The desire to integrate data, voice, image and video over high-speed digital trunks has led to the development of a variety of packet and cell switching techniques. One such technique is called Asynchronous Transfer Mode (ATM). ATM is a switching technology that provides users with the ability to connect to one or more users in a transparent fashion. Unlike the variable length packets used by frame relay services, ATM service is based on switching fixed length packets of data known as cells. Cell switching, as it is called, is gaining popularity for a variety of reasons. First, switch architectures can be optimized to switch cells at much higher speeds than variable length packets. Second, multiple services requiring a variety of qualities of service guarantees can be provided simultaneously. ATM user traffic is first segmented into fixed length cells, transmitted, then reassembled back into its original form. This segmentation and reassembly (SAR) process is done in a standardized way, regardless of the carrier providing the ATM service.
Although the use of fixed length cells in ATM can be efficient in terms of allowing standardized switching apparatus to be used, for many applications the standard 53-byte cell provides too large a package for the data (e.g., compressed voice) requiring transport through the network. As a result, much of the cell payload is merely “padding” and the transport of such padding wastes the available bandwidth of the ATM network. Several approaches to solve this problem present themselves.
For example, one could use a shorter length cell. As indicated, ATM uses a standard 53-byte cell, with 48-bytes of payload and 5-bytes of header information. Choosing a smaller cell size could result in less of the cell payload being filled by padding. However, shorter cells have two important disadvantages. First, such cells would be non-standard and, as a result, such cells could not be transported through ATM networks designed to accommodate only standard size cells. This lack of interoperability would likely mean that users would be disinclined to accept such a solution. Second, smaller cells would likely end up wasting more bandwidth than they would save because the ratio of header size to payload is much higher than for a 53-byte cell. Unfortunately, the header size could probably not be reduced from the current 5-byte size without a loss of functionality.
Another solution might be to use variable length packets, as is common in frame relay networks. This could conceivably avoid the need for padding altogether because packets could be “custom built” to the requirements of the user data. Unfortunately, the very fact that such varying packet sizes are allowed within frame relay networks means that the switches used to transport the packets across the network must be more complex than their ATM counterparts. As a result, such switches are generally slower than ATM switches. Further, in networks where variable length packets are used, it is difficult to make real time service guarantees without the use of complex servicing and queuing algorithms and some limitations on packet size.
Yet another solution might be to pack multiple payloads into one cell at a source and then pull these payloads apart at a destination. The ATM Forum has promulgated a standard regarding such bundling of data channels within a single cell, entitled “ATM Trunking Using AAL2 for Narrowband Services”, AF-VOTA-0113.000 (February 1999), which is incorporated herein by reference. AAL2 (ATM Adaptation Layer type 2) provides a standards-based approach for multiplexing multiple voice channels on a single virtual circuit (VC) between two points. Thus, it provides for conserving bandwidth where the use of standard ATM cells might be wasteful. For this reason, AAL2 is a desirable solution for bringing ATM service to customer premises.
However, carriers wishing to bring such services up to the customer premises face other challenges. For example, existing ATM hardware (e.g., switches and the like) cannot accommodate the sub-multiplexed packets that make up an AAL2 cell. Furthermore, the installed base of signaling protocols (i.e., the process by which ATM users and the network exchange control information, request the use of network resources, or negotiate for the use of circuit parameters) are designed for use with 53-byte ATM cells and will not readily accommodate multiple AAL2 sub-cells. Accordingly, what is needed is a scheme for allowing the use of switched AAL2 up to the customer premises using a protocol set for networking the AAL2 connections which is fully consistent with those that are used to manage connection set ups across a regular ATM network.